The present invention relates to a heavy duty tire comprising bead portions having an improved durability.
FIG. 8 illustrates a sectional view of an example of a bead portion of a conventional heavy duty tire. Such a heavy duty tire includes a carcass ply “b” made of steel cords. The carcass ply “b” includes a toroidal main body portion b1, and turnup portions b2 that continue to the main body portion b1 and are wound along the periphery of bead cores “c”. The turnup portions b2 are wound around the bead cores “c” so that their terminal ends b2e are located in the proximity of the radially outer surface of the bead cores “c”. Such turnup portions b2 can decrease damages such as cord loosening starting from their terminal ends b2e, since the terminal ends b2e of the turnup portions b2 are disposed in regions that show small distortions even when the tire is running in a loaded condition.
Heavy duty tires are filled with air of extremely high pressure of about 600 to about 800 kPa. As a result, cords of the turnup portions b2 of the carcass ply “b” receives a force F that pulls them toward the main body portion b1 (hereinafter, such force is also simply referred to as “pulling out force”). Such a pulling out force F acts to rotate the bead cores “c” surrounded by the turnup portions b2 in a direction as indicated by the arrow in FIG. 8. This consequently leads to unintended deformation of the sectional shape of the bead cores “c” or lifting deformation of bead toes Bt from seat surface Js of rim J. Such deformations deteriorate the durability of the bead portions and the air-in performance of retread tires. The air-in performance is indicative of the easiness of air-filling operations conducted when a tire is mounted on a rim, and it is largely affected by the shape of the beads. If the air-in performance is poor, air is easy to leak out from a clearance formed between the bead and the rim.
It is an object of the present invention to provide a heavy duty tire wherein the rotation deformation of the bead cores and other deformation caused by filling of air can be repressed over a long period of time to thereby improve the durability of the bead portions.
This and other objects of the present invention will become apparent from the description hereinafter.